The small molecule chemical residues (or solvent residues) in the packaging materials seriously endanger the health of consumers, which cause the attention of the food safety management organizations of the countries around the world; so, the development of the food packaging materials which remains the low amount of small molecule chemical residues has become a research focus of the scholars from various countries.
In recent years, the accidents of food contamination caused by the packaging materials have occasionally occurred; the main reason of causing food contamination is the small molecule chemicals which exist in the packaging materials; the small molecule chemical pollutants can be organic solvents, initiators, plastic processing aids and the like in printing inks or binders. The odor of the packaging materials is one of the main reasons of causing food contamination; and the odor is mainly generated by the organic solvents. One of the main sources of the organic solvents is the solvent-based inks used when producing the flexible package; ethyl acetate is the main solvent of the solvent-based inks; a large part of the flexible packages are produced by adopting the dry laminating process at present; a solvent-based polyurethane binder is required in the production process, and such polyurethane binder uses toluene, ethyl acetate and the like as the organic solvents. However, due to the performance of the materials used in the flexible packaging materials, the solvents will not volatize in a timely manner when passing through a drying oven. The Low Density Polyethylene (LDPE) film and the Casting Polypropylene (CPP) film have heat-sealing properties; the Biaxially Oriented Polypropylene (BOPP) film has relatively excellent transparency, high gloss and printing performance; and as the polyolefin films are low in cost, the consumption of the polyolefin films used in the flexible packaging materials is more than 50%. Recently, almost all of the flexible packages are multi-layer laminated films and are implemented with color printing, thus causing the food safety hazards.
The principle of “like dissolves like” is a general rule shown by the polymer material when being dissolved in the organic solvents, and this principle is also suitable for the solvent residue phenomenon caused by the packaging materials absorbing the organic solvents. As known to all, the solubility parameters (δ) of the Polyethylene (PE) and the Polypropylene (PP) flexible packaging materials are respectively 18.9 and 16.4 (J/cm3)1/2; the solubility parameters of the two organic solvents, namely, the ethyl acetate and toluene, are respectively 18.6 and 18.2 (J/cm3)1/2; the solubility parameters of the PE and PP materials films are quite similar with the solubility parameters of the two general organic solvents, namely, the ethyl acetate and toluene; therefore, the toluene and ethyl acetate must have a relatively high solubility in the polyolefin flexible packaging materials, namely, the polyolefin films have quite high absorbability to the organic solvents, and can absorb a large number of organic solvents; compared with the Polyethylene Terephthalate (PET) (δ=21.9) with relatively high polarity, the flexible packaging materials which contain the polyolefin films are easier to cause excessive solvent residue; and this is proved by the practices of the flexible package manufacturing factory; this fact can be explained as there is a large difference between the solubility parameters of the PET materials and the solubility parameters of toluene and ethyl acetate.
Many scholars have researched a large number of qualitative and quantitative detection methods for the organic solvent residues in the packaging materials; however, the problems that which processing technology can be adopted to reduce the solvent residues in the packaging materials, and what structure of the PE and PP films have small solubility to the organic solvents are seldom researched. At present, the flexible package manufacturing enterprises reduce the solvent residues of the flexible packaging products by changing the drying conditions, adjusting the printing or lamination speed, changing the printing technologies and the like; the flexible packaging materials with excessive solvent residue value can be remedied via the methods of ventilating, increasing curing time, increasing the rewinding times and the like; however, the methods used for reducing the solvent residues of the packaging materials are passive; once adopting the PE and PP packaging materials, and the solvents such as toluene and ethyl acetate, the problems of solvent residue will be always existent, and this is an inevitable result caused by the “like dissolves like” principle.
The consumption of the packaging materials is increased at a rate of about 10% every year around the world, the consumption of the PE and PP resins for packaging is increased accordingly; however, the problem that the polyolefin film has a relatively high solvent residue value always cannot be solved by the flexible packaging manufacturing enterprises. In order to enhance the food safety, each of the countries has issued more serious standards of limiting the solvent residues, which sets a higher “threshold” for the flexible package enterprises.
Recently, food contamination accidents have been caused by small molecule chemicals remaining in the package. Such contamination endangers the health of consumers and may cause economic loss to the manufacturers. Thus, the work of researching the residue mechanism of the organic solvents in the polyolefin, researching what polyolefin with the suitable aggregation structure has relatively low toluene and ethyl acetate absorbability, and researching which processing technology can be adopted to prepare the flexible packaging materials, which have low solvent residues and can better satisfy the food safety requirements, has very important theoretical significance and practical application value.
In the field of printing and packaging, the PE and PP films have lots of problems during the use, for example, poor printing performance, low peeling strength of the laminated films, higher solvent residue vale and the like; aiming at the above problems, the Chinese patent application 200810171239.X (Publication No. CN101724176A) discloses a polyolefin processing aid for food flexible packaging, a preparation method thereof, the use, and a polyolefin modified by the processing aid, and discloses a nano-silicon dioxide/Ethylene-Vinyl Acetate (EVA) polyolefin modification additive. In the Chinese patent application No. 200910176114.0 and 200910176112.1, the PP flexible packaging materials and the PE flexible packaging materials modified by the above silicon dioxide/EVA are respectively adopted.
However, the existing processing aids need to be further improved in the aspect of reducing the solvent residues.